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| United States Patent |
5,685,923
|
|
Takata
, et al.
|
November 11, 1997
|
Ferritic stainless steel bellows
Abstract
Disclosed is a ferritic stainless steel for forming into bellows having
high resistance to stress corrosion cracking, comprising, in terms of wt
%, not greater than 0.02% of C, from 0.1 to 1.5% of Si, not greater than
1.0% of Mn, from 11.0 to 22.0% of Cr, from 0.01 to 0.08% of Al, not
greater than 0.015% of N, at least one of not greater than 0.6% of Ti and
not greater than 1.0% of Nb, whenever necessary, and furthermore, at least
one of not greater than 2% of Mn, not greater than 1.5% of Cu and not
greater than 1.5% of Ni. This steel has the crystal grain size measured in
a section in a direction orthogonal to a rolling direction is not greater
than 8.5 in terms of the grain size number, and the crystal grain size
measured in a section orthogonal to a section parallel to the rolling
direction is not smaller than 5.0 on an average in terms of the grain size
number.
| Inventors:
|
Takata; Ken (Futtsu, JP);
Yamamoto; Akio (Futtsu, JP)
|
| Assignee:
|
Nippon Steel Corporation (Tokyo, JP)
|
| Appl. No.:
|
579187 |
| Filed:
|
December 27, 1995 |
Foreign Application Priority Data
| Current U.S. Class: |
148/325 |
| Intern'l Class: |
C22C 038/06; C22C 038/18 |
| Field of Search: |
420/34,70
148/325
|
References Cited
U.S. Patent Documents
| 4286986 | Sep., 1981 | Borneman et al. | 148/325.
|
| 4461811 | Jul., 1984 | Borneman et al. | 420/68.
|
| 4690798 | Sep., 1987 | Narutani et al. | 420/61.
|
| Foreign Patent Documents |
| 49-107915 | Oct., 1974 | JP.
| |
Primary Examiner: Yee; Deborah
Attorney, Agent or Firm: Kenyo & Kenyon
Claims
We claim:
1. Bellows made of a ferritic stainless steel consisting essentially of, in
terms of wt %:
C: not greater than 0.02%,
Cr: from 11.0 to 22.0%,
Al: from 0.01 to 0.08%,
N: not greater than 0.015%, and
balance Fe and unavoidable impurities,
wherein the crystal grain size measured in a section in a direction
orthogonal to a rolling direction is not greater than 8.5 in terms of the
grain size number, and the crystal grain size measured in a section
orthogonal to a section parallel to the rolling direction is not smaller
than 5.0, on average, in terms of the grain size number.
2. Bellows made of a ferritic stainless steel consisting essentially of, in
terms of wt %:
C: not greater than 0.02%,
Si: from 0.1 to 1.5%,
Mn: not greater than 1.0%,
Cr: from 11.0 to 22.0%,
Al: from 0.01 to 0.08%,
N: not greater than 0.015%,
balance Fe and unavoidable impurities, and
at least one of the following components:
Ti: at least four times the sum of the C and N contents and not greater
than 0.6%, and
Nb: at least eight times the sum of the C and N contents and not greater
1.0%,
wherein the crystal grain size measured in a section in a direction
orthogonal to a rolling direction is not greater than 8.5 in terms of the
grain size number, and the crystal grain size measured in a section
orthogonal to a section parallel to the rolling direction is not smaller
than 5.0, on average, in terms of the grain size number.
3. Bellows made of a ferritic stainless steel consisting essentially of, in
terms of wt %:
C: not greater than 0.02%,
Cr: from 11.0 to 22.0%,
Al: from 0.01 to 0.08%, and
N: not greater than 0.015%,
balance Fe and unavoidable impurities, and
at least one of the following components:
Mo: not greater than 2%,
Cu: not greater than 1.5%, and
Ni: not greater than 1.5%,
wherein the crystal grain size measured in a section in a direction
orthogonal to a rolling direction is not greater than 8.5 in terms of the
grain size number, and the crystal grain size measured in a section
orthogonal to a section parallel to the rolling direction is not smaller
than 5.0, on average, in terms of the grain size number.
4. Bellows made of a ferritic stainless steel consisting essentially of, in
terms of wt %:
C: not greater than 0.02%,
Cr: from 11.0 to 22.0%,
Al: from 0.01 to 0.08%, and
N: not greater than 0.015%, and
at least one of the following components:
Ti: at least four times the sum of the C and N contents and not greater
than 0.6%, and
Nb: at least eight times the sum of the C and N contents and not greater
1.0%, and
at least one of the following components:
Mo: not greater than 2%,
Cu: not greater than 1.5%, and
Ni: not greater than 1.5%,
and balance Fe and unavoidable impurities,
wherein the crystal grain size measured in a section in a direction
orthogonal to a rolling direction is not greater than 8.5 in terms of the
grain size number, and the crystal grain size measured in a section
orthogonal to a section parallel to the rolling direction is not smaller
than 5.0, on average, in terms of the grain size number.
5. Bellows made of a ferritic stainless steel according to any of claims 1
through 4, wherein the crystal grain size measured in the section in the
direction orthogonal to the rolling direction is not greater than 8.0 in
terms of the grain size number, and the crystal grain size measured in the
section orthogonal to the section parallel to the rolling direction is not
smaller than 5.0, on average, in terms of the grain size number.
6. Bellows made of a ferritic stainless steel according to any of claims 1
through 4, wherein the crystal grain size measured in the section in the
direction orthogonal to the rolling direction is not greater than 7.5 in
terms of the grain size number, and the crystal grain size measured in the
section orthogonal to the section parallel to the rolling direction is not
smaller than 5.0, on average, in terms of the grain size number.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ferritic stainless steel for forming
into bellows.
In comparison with bellows made of an austenitic stainless steel, bellows
made of a ferritic stainless steel have the characterizing features that
stress resistance and corrosion crack resistance are extremely excellent.
However, forming into the bellows is extremely difficult, and breakage
frequently occurs during forming. The present invention relates to a
ferritic stainless steel for forming into bellows, capable of reducing the
amount of breakage which is likely to occur during forming.
2. Description of the Prior Art
In various machines and apparatuses handling gasses, solutions and powders,
the materials are mostly transferred through metal pipings. Bellows are
used at intermediate portions in such metal pipings so as to absorb the
strain resulting from thermal expansion and vibration, and to prevent
transmission of the strain and the vibration. Conventionally, copper, an
austenitic stainless steel, etc., have been used for the bellows because
forming other metals into a bellows structure has been difficult. In other
words, copper and the austenitic stainless steel have large elongation
while cold and are the most optimum materials for the bellows to which
bulge processing, formed by elongation, is particularly applied. In
contrast, those metals which comprise bcc crystals such as carbon steel
cannot be processed by bulging because the ductility of the metals,
particularly the ductility at a weld portion, is not sufficient.
On the other hand, though the bellows made of the austenitic stainless
steel can be easily produced, they involve the problem that stress
corrosion cracks are likely to occur due to corrosive solutions passing
through the bellows. The bellows absorb the stress and the vibration by
bending at the protruding peak portions and the recessed valley portions
thereof and for this reason, the stress always acts on the protruding
portions and the recessed portions. In other words, the bellows have a
structure or a component from which the stress can never be removed.
Nonetheless, austenitic stainless steel is an alloy having high stress
corrosion cracking susceptibility. Accordingly, bellows made of the
austenitic stainless steel involve a problem that stress corrosion
cracking is extremely likely to occur.
To avoid stress corrosion cracking, there are only two methods, that is,
either a material having low stress corrosion crack susceptibility is used
or a structure in which stress corrosion crack cannot easily occur, that
is, a structure that does not leave a stress load, is employed. To reduce
stress corrosion crack susceptibility of the austenitic stainless steel,
Japanese Unexamined Patent Publication (Kokai) No. 49-107915, for example,
proposes to reduce the Cr, N, Mo and P content by increasing the Ni
content. Even when such a steel is employed, however, only the time before
the occurrence of stress corrosion crack is prolonged to a certain extent,
but the occurrence of stress corrosion cracking cannot be prevented.
On the other hand, it would be conceivable to reduce the stress acting on
the recessed portions or on the protrusive portions by increasing the
number of concavo-convexities or by reducing the bending angle of the
recessed portions or the protrusive portions so as to disperse the stress.
According to this method, however, the bellows become elongated or large,
so that the apparatus must be greater in size and the cost of production
becomes higher. Nonetheless, the problem of susceptibility to stress
corrosion cracking cannot be solved, and the problem of stress corrosion
cracking has remained unsolved depending on environments.
In contrast, the present inventors have succeeded in producing bellows
which practically eliminate stress corrosion cracking susceptibility by
limiting ductility of the ferritic stainless steel.
SUMMARY OF THE INVENTION
Although the present inventors have thus succeeded in machining the
ferritic stainless steel into the bellows, breakage, which as results from
an insufficiency of ductility of the material, occurs many times during
forming particularly at the peak portions of the bellows and at the end
portions of the formed portion, and the yield has been much lower than
that from machining austenitic stainless steel. Therefore, the present
inventors have conducted experiments so as to improve the ductility of the
raw material by reducing the C and N contents to the minimum contents on
the basis of a known concept. Although ductility of the raw material, by
the tensile test, has been improved, breakage at the peak portions of the
bellows and at the end portions of the formed portion have not necessarily
been reduced.
Accordingly, when the fracture is inspected, Al and O are detected at a
part of the fracture, and it has thus been found out that aluminum oxides
are associated with the breakage. It has been deduced from this fact that
the breakage starts from inclusions and develops into ductile breakage. It
has been recognized in the past that when the workpiece is bent,
particularly parallel to a rolling direction, sulfide type inclusions
which extend along the rolling direction function as the starting point of
the breakage due to bending, and spherical alumina type inclusions were
believed harmless unless they were particularly coarse. According to the
tensile test, further, ductility is not lowered unless the alumina type
inclusions are particularly coarse.
If the alumina type inclusions are the cause of the breakage,
counter-measures can be taken by reducing the Al content. Therefore, when
the amount of addition of Al is reduced to 0.005% or below in terms of
acid-soluble Al, deoxidation becomes incomplete and large quantities of Si
type inclusions occur, though the Al type inclusions can be eliminated.
Though forming of this material into the bellows has not been done, it is
believed to be unavoidable, according to the past experience, that
machining cracks develop. In other words, when Al is added, the alumina
type inclusions occur and when the Al content is reduced, deoxidation
becomes incomplete and large quantities of the Si type inclusions occur.
In either case, cracks resulting from the inclusions cannot be prevented.
As described above, the present inventors have encountered with phenomena
which are different from the impediments to the conventional bellows
processing, and the conventional concept cannot cope with such
impediments.
Accordingly, the present invention contemplates prevention of machining
cracking due to the inclusions different from the conventional concept,
and to improve the yield and productivity of the ferritic stainless steel
in the bellows.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present inventors have conducted various analyses of the conditions of
the occurrence of breakage during forming of ferritic stainless steel into
bellows, and have found out that the degree of occurrence of cracking is
relatively mild in materials whose final annealing temperature is high. It
is believed that the strength of the material is lowered and its ductility
can be improved if the final annealing temperature is high, and a high
final annealing temperature may be qualitatively advantageous for reducing
the cracking during forming. However, when the mechanical characteristics
of the materials are examined by the tensile test, the level of the
reduction of the strength and the level of the improvement in ductility
are not always significant.
On the other hand, the portions of the occurrence of breakage at the time
of forming the ferritic stainless steel into the bellows have been
examined in detail. As a result, it has been found out that when the
alumina type inclusions exist in the grain boundary, they are extremely
likely to become the starting point of breakage. When this fact is taken
into consideration in combination with the fact described above, that the
occurrence of crack becomes less when the final annealing temperature of
the material is higher, it is assumed that the higher the final annealing
temperature, the coarser becomes the crystal grain and, eventually, the
quantity of the alumina inclusions existing in the grain boundary
decrease, so that the occurrence ratio of breakage occurring during
forming into the bellows is reduced.
The reason why the inclusions existing in the grain boundary exert great
influence on breakage occurring during forming into the bellows has not
been clarified. However, because forming into the bellows is not a
deformation in one direction such as a tensile test or bending but is
two-dimensional process which also involves deformation in a planar
direction, the difference of this deformation mode would presumably exert
an influence.
From the result described above, it is estimated that when any inclusions
other than the alumina type inclusions exist in the grain boundary,
breakage is more likely to occur during forming of the bellows.
Accordingly, to prevent the occurrence of breakage, it is very important
to reduce, as much as possible, the inclusions and, at the same time, to
reduce the area of the grain boundary.
According to the prior concept, inclusions extending in the rolling
direction are detrimental to elongation (bending) in a direction at right
angles to the inclusions. When deoxidation of the steel is not sufficient,
Si type stretched inclusions are generated. Therefore, it is important to
reliably carry out deoxidation and to this end, the addition of Al is
indispensable.
The technical concept of the present invention is directed to preventing
cracks occurring during forming into the bellows by reducing the
probability of the existence of the inclusions in the grain boundary by
increasing the crystal grain size on the basis of the concept described
above. A bulging which is used for forming into the bellows imparts
two-dimensional deformation to the raw material, but in forming of the
steel pipe into the bellows, deformation in a direction orthogonal to the
rolling direction is particularly great. On the other hand, because the
ferritic stainless steels generally have the crystal grains stretched in
the rolling direction, the probability of the existence of the inclusions
in the crystal grain which is substantially parallel to the rolling
direction is believed particularly great. Accordingly, the present
invention does not merely reduce the probability of the existence of the
inclusions in the grain boundary by increasing the crystal grain size, but
is mainly directed to reduce the crystal grain boundary which is parallel
to the rolling direction.
When various examinations are conducted on the basis of the technical
concept described above, cracks occurring during forming into the bellows
can be drastically reduced by limiting the crystal grain size, which is
measured in the section in a direction orthogonal to the rolling
direction, to 8.5 or below, preferably not greater than 8.0 and most
preferably not greater than 7.5, in terms of the grain size number. The
grain size number is defined under the ASTM standard.
The method of controlling the grain size measured in the section orthogonal
to the rolling direction and the grain size measured in the section
parallel to the rolling direction can be practiced by expanding and
combining the conventional concepts.
Recrystallization of the cold rolled materials occurs in such a manner that
the crystal grains before cold rolling are stretched in the rolling
direction by cold rolling and the crystal grains so stretched are then cut
by subsequent annealing and are allowed to undergo divided
recrystallization. If the crystal grain is great before cold rolling or
the reduction is small, in this instance, the crystal grain size measured
in the section orthogonal to the rolling direction becomes great.
When the starting point of recrystallization is reduced by lowering the
annealing temperature, the crystal grain size measured in the section
parallel to the rolling direction becomes great. When the annealing
temperature becomes even lower, the crystal grains do not undergo
recrystallization but are merely softened. In this case, the crystal
grains are not different from the crystal grains as rolled in the rolling
direction and as cold rolled.
Accordingly, it becomes possible to control the crystal grain size measured
in the section orthogonal to the rolling direction by selecting a suitable
reduction ratio in accordance with the crystal grain size before cold
rolling, and to control the crystal grain size measured in the section
parallel to the rolling direction by setting the annealing temperature to
a suitable temperature.
In other words, the grain size measured in the section orthogonal to the
rolling direction and the grain size measured in the section parallel to
the rolling direction can be suitably controlled by suitably selecting the
crystal grain size before cold rolling, the cold rolling reduction ratio
and the annealing temperature.
The present invention has been completed on the basis of the technical
concept described above. Firstly, the present invention provides:
a ferritic stainless steel for forming into bellows comprising, in terms of
wt %:
C: not greater than 0.02%
Cr: 11.0 to 22.0%,
Al: 0.01 to 0.08%,
N: not greater than 0.015%,
wherein the crystal grain size measured in a section in a direction
orthogonal to a rolling direction is not greater than 8.5 in terms of the
grain size number, and the crystal grain size measured in a section
orthogonal to a section parallel to the rolling direction is not smaller
than 5.0 on an average in terms of the grain size number.
When the temperature of use of the bellows is within the range of
600.degree. to 900.degree. C., carbonitrides of Cr that have been
precipitated in the raw material at normal temperature again undergo solid
solution, to again precipitate at the grain boundary, become further
coarser, lower the strength of the raw material and serve as the starting
points of fatigue and corrosion fatigue. This reprecipitation can be
eliminated by fixing the Cr carbonitrides to those carbides or nitrides
which do not undergo solid solution within this temperature range, at the
stage of the raw material during the production. Therefore, the present
inventors have attempted to convert C and N to Ti and Nb carbonitrides
which are stable at high temperature, by adding Ti and Nb.
When the temperature is within such a high range, various salts existing in
the atmosphere, such as in an exhaust system of automobiles, adhere and
are fused to generate molten salt corrosion. In this case, so-called
"molten salt corrosion resistance" is required in addition to oxidation
resistance. It has been found out that the addition of a suitable amount
of Si to form a stable Si oxide film is effective for preventing this
molten salt corrosion.
Secondary, the present inventors have completed the second concept as one
of the embodiments directed to use in an environment of 600.degree. to
900.degree. C. such as in the exhaust system of automobiles. The invention
provides:
a ferritic stainless steel for forming into bellows comprising, in terms of
wt %:
C: not greater than 0.02%,
Si: from 0.1 to 1.5%,
Mn: not greater than 1.0%,
Cr: from 11.0% to 22.0%,
Al: from 0.01 to 0.08%,
N: not greater than 0.015%,
at least one of the following components:
Ti: at least four times the sum of the C and N contents and not greater
than 0.6%, and
Nb: at least eight times the sum of the C and N contents and not greater
than 1.0%, and
other unavoidable impurities and Fe,
wherein the crystal grain size measured in a section in a direction
orthogonal to a rolling direction is not greater than 8.5 in terms of the
grain size number, and the crystal grain size measured in a section
parallel to the rolling direction is not smaller than 5.0 on an average in
terms of the grain size number.
Thirdly, the bellows are often used as a component for apparatuses handling
chemicals. In this case, heretofore known chemical resistance improving
elements can be added so as to satisfy the requirement for the corrosion
resistance. The third and fourth inventions are completed as an embodiment
for the application for which a high corrosion resistance is required, in
line with the concept described above. The present invention further
provides:
a ferritic stainless steel for forming into bellows comprising; in terms of
wt %,
C: not greater than 0.02%,
Cr: from 11.0 to 22.0%,
Al: from 0.01 to 0.08%,
N: not greater than 0.015%,
at least one of the following components,
Mo: not greater than 2%,
Cu: not greater than 1.5%, and
Ni: not greater than 1.5%,
wherein the crystal grain size measured in a section in a direction
orthogonal to a rolling direction is not greater than 8.5 in terms of the
grain size number, and the crystal grain size measured in a section
orthogonal to a section parallel to the rolling direction is not smaller
than 5.0 on an average in terms of the grain size number.
Fourthly, the present invention provides; a ferritic stainless steel for
forming bellows comprising, in terms of wt %,
C: not greater than 0.02%,
Si: from 0.1 to 1.5%,
Mn: not greater than 1.0%,
Cr: from 11.0 to 22.0%,
Al: from 0.01 to 0.08%,
at least one of the following components,
Ti: at least four times the sum of the C and N contents and not greater
than 0.6%, and
Nb: at least eight times the sum of the C and N contents and not greater
than 1.0%,
at least one of the following components,
Mo: not greater than 2%,
Cu: not greater than 1.5%,
Ni: not greater than 1.5%, and
other unavoidable impurities and Fe,
wherein the crystal grain size measured in a section in a direction
orthogonal to a rolling direction is not greater than 8.5 in terms of the
grain size number, and the crystal grain size measured in a section
orthogonal to a section parallel to the rolling direction is not smaller
than 5.0 on an average in terms of the grain size number.
Next, the limiting conditions in the present invention will be explained.
The ferritic stainless steel for the bellows is limited to those ferritic
stainless steels in which C is limited to not greater than 0.02% and N, to
not greater than 0.015%.
The ferritic stainless steel has far higher resistance to stress corrosion
cracking than the austenitic stainless steel, irrespective of its Cr
content, in a bellows. However, if the Cr content is less than 11%, the
basic corrosion resistance becomes extremely low, and if it is added in a
large amount, the machinability deteriorates. Therefore, the upper limit
is set to 22.0%.
If the C content of the raw material exceeds 0.02%, forming of the steel
into the bellows becomes difficult and even if forming can be done, the
fatigue characteristics deteriorate due to the Cr carbides precipitating
in the raw material. For this reason, the upper limit is set to 0.02%.
If the N content in the raw material exceeds 0.015%, forming into the
bellows becomes difficult in the same way as in the case of C and even if
forming can be done, the fatigue characteristics deteriorate due to the Cr
nitrides precipitating in the raw material. Therefore, the upper limit is
set to 0.015%.
Aluminum (Al) is the element necessary for deoxidation. Since deoxidation
must be reliably effected, the lower limit is set to 0.01%. If Al is added
in a large amount, the viscosity of a solution becomes high, floating of
alumina type inclusions as the deoxidation product is limited and the
inclusions are likely to remain as such. Moreover, ductility of the raw
material is lowered. For these reasons, the upper limit is set to 0.08%.
To reduce the crystal grain boundary, particularly the grain interface
substantially parallel to the rolling direction, the crystal grain size
measured in the section in an orthogonal direction to the rolling
direction is limited to not greater than 8.5, preferably not greater than
8.0 and further preferably not greater than 7.5, in terms of the grain
size number, and in this way, cracks that occur during forming of the
steel into the bellows can be drastically reduced. Therefore, the upper
limit is set to 8.0. However, if the grain size is too great, the
concavoconvexities occurring in processing become great and the fatigue
characteristics during use deteriorate. Therefore, the lower limit is set
to not smaller than 5.0 on an average as the grain size number measured in
the section (C section) orthogonal to the section (L section) parallel to
the rolling direction.
In the second invention, as an embodiment directed to the use in an
environment of 600.degree. to 900.degree. C. such as in an exhaust system
of automobiles, the addition of Si constitutes a characterizing feature.
To secure the molten salt corrosion resistance, a stable oxide film of Si
must be formed within a temperature range of 600.degree. to 900.degree. C.
and to this purpose, at least 0.1% of Si must be added. However, if Si is
added in a large amount, forming of the steel into bellows becomes
extremely difficult, and even if the forming can be done, the difference
between the protrusive portions and the recessed portions becomes so small
that the structure must be extremely elongated in order to secure the
bellows functions. Therefore, the upper limit is set to 1.5%.
Although Mn is necessary to secure machinability of the raw material when
forming it into the bellows, the upper limit of Mn is set to 1.0% because
Si is added to secure the resistance to molten salt corrosion.
The third invention, which is completed as the embodiment not requiring a
severe limitation on welding methods and devoid of degradation of the
fatigue characteristics and the corrosion fatigue characteristics due to
coarsening of the precipitates during the use, is characterized in that Ti
and Nb are added.
When Ti undergoes solid solution at the time of welding, it fixes the major
proportions of C and N, and its equivalent is at least the four times the
sum of the C and N contents. Therefore, this value is set as the lower
limit. When Ti is added in a large amount, however, it absorbs N and forms
the nitride at the time of welding, and a limitation on the welding method
becomes necessary again. Therefore, the upper limit is set to 0.6%.
When Nb undergoes solid solution at the time of welding, it fixes the major
proportions of C and N, and its equivalent is at least eight times the sum
of the C and N contents. Therefore, this value is set as the lower limit.
When Nb is added in a large amount, however, it absorbs N and forms the
nitride at the time of welding, and a limitation on the welding method
becomes necessary again. Therefore, the upper limit is set to 1.0%.
The fourth invention, as embodiments directed to an application for which
high corrosion resistance is required, are characterized in that at least
one of Mo, Cu and Ni is added.
Mo is particularly effective for the chlorine ion but when it is added in
an amount exceeding 2%, machinability drops and machining into bellows
becomes difficult.
Cu improves the corrosion resistance particularly in a low pH environment
containing sulfuric acid, but when it is added in an amount exceeding
1.5%, Cu which is not converted to the solid solution, precipitates in the
grain boundary of the weld portion and forming into the bellows becomes
difficult. Therefore, this value is set as the upper limit.
Ni improves the corrosion resistance in a low pH environment, but when it
is added in an amount exceeding 1.5%, a martensite phase occurs at the
weld portion and forming into bellows becomes difficult. Therefore, this
value is set as the upper limit.
The ferritic stainless steel as the base metal can sufficiently secure the
difference of the diameters between the protruding portions and the
recessed portions of the bellows by limiting the C and N contents to low
levels.
Deoxidation is reliably carried out by limiting Al to reduce the
inclusions, and at the same time, the inclusions which exist in the grain
boundary and become the cause of the breakage occurring during forming of
the ferritic stainless steel into the bellows are reduced by limiting the
area of the grain boundary parallel to the rolling direction which is
particularly detrimental. As a result, the grain boundary which opens due
to two-dimensional deformation and has a low strength can be reduced and,
eventually, breakage during forming of the bellows can be reduced.
Because a suitable amount of Ti is added to the base metal, precipitation
of the Cr carbides at the weld portion and the heat affected portions can
be eliminated, and the occurrence of grain boundary corrosion and
deterioration of the full face corrosion resistance can also be
eliminated. Accordingly, measures for minimizing the fused portions and
the heat affected portions by limiting the welding method becomes
unnecessary, so that severe limitations on the welding method are not
necessary.
Further, because a suitable amount of Ti is added, the carbonitrides of Cr
in the raw material which precipitate in an environment of 600.degree. to
900.degree. C. again undergo solid solution and consequently, a drop in
the strength of the raw material and a deterioration of the fatigue
characteristics resulting from re-precipitation of the Cr carbonitrides to
the grain boundary can be eliminated. Because a suitable amount of Si is
added and the Cr content is limited, a stable Si oxide film can be formed
in addition to the oxide film consisting principally of Cr. Therefore, the
oxidation resistance can be improved and the so-called molten salt
corrosion resistance, to molten salt corrosion occurring due to adhesion
and melting of various salts existing in the environment, can be improved.
As a result, forming of the ferritic stainless steel into the bellows
having excellent high temperature characteristics of 600.degree. to
900.degree. C., such as in the exhaust system of automobiles, can be done.
As described above, the present invention can drastically improve the
production yield of bellows from the economical ferritic stainless steel
without containing expensive Ni and having excellent stress corrosion
crack resistance. Conventionally, although the ferritic stainless steel
has been formative into the bellows, breakage has occurred more frequently
during forming than in the austenitic stainless steel, and the production
cost has been higher. However, the present invention can drastically
improve the production yield, and the advantage that it does not contain
Ni reduces the production cost.
The steel according to the present invention makes it possible to produce
the bellows sufficiently satisfactory in both high temperature fatigue and
molten salt corrosion, in addition to stress corrosion cracking, even when
used in a high temperature environment of 600.degree. to 900.degree. C.
such as in an exhaust system of automobiles.
As a result, maintenance of apparatuses using the bellows can be extremely
simplified, and service life of the apparatuses can be extended as a
whole. Because maintenance can be thus simplified, productivity can be
improved. Therefore, the present invention provides great industrial
advantages.
EXAMPLE
Example 1
Electric welded steel pipes having an outer diameter of 55 mm.phi. were
produced by using 0.7 mm-thick cold rolled sheets (annealed materials) of
the steels tabulated in Table 1. In this instance, the crystal grain of
each cold rolled annealed material was controlled by limiting a cold
rolling reduction ratio and an annealing temperature after cold rolling. A
production test of bellows having a peak pitch of 15 mm, three peak
heights of 10, 11 and 12 mm, a total number of peak of 18 and a full
length of about 250 mm was carried out by hydraulic bulging from each of
the weld pipes. Table 2 shows the relationship between the result and each
grain size of the raw material.
TABLE 1
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chemical composition
steel
C Si Mn Cr Ni Ti Nb Mo Cu Al N Remarks
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A 0.007
0.26
0.11
16.3
-- -- -- -- -- 0.021
0.0060
steel within the range
of the invention
B 0.006
1.26
0.14
16.4
-- 0.16
-- -- -- 0.027
0.0053
steel within the range
of the invention
C 0.011
0.85
0.66
13.8
-- 0.14
0.22
-- -- 0.014
0.0073
steel within the range
of the invention
D 0.006
0.23
0.11
12.6
0.35
0.10
-- -- -- 0.016
0.0049
steel within the range
of the invention
E 0.008
0.33
0.15
17.1
-- 0.18
-- 1.30
-- 0.018
0.0074
steel within the range
of the invention
F 0.010
0.48
0.18
19.2
0.26
-- 0.35
-- 0.42
0.013
0.0096
steel within the range
of the invention
G 0.035
0.45
0.48
16.2
-- -- -- -- -- 0.108
0.0113
comparative steel
H 0.009
0.32
0.88
13.5
-- 0.18
0.26
-- -- 0.045
0.0181
comparative steel
I 0.009
0.34
0.15
16.1
-- -- -- -- -- 0.001
0.0133
comparative steel
__________________________________________________________________________
TABLE 2
______________________________________
C sec- L sec-
tion tion means existence of
grain grain grain machining cracks
No steel size size size 10 mm 11 mm 12 mm
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1 A 7.3 6.3 6.8 .smallcircle.
.smallcircle.
.smallcircle.
2 B 8.3 7.8 8.1 .smallcircle.
x x
3 B 7.4 6.0 6.7 .smallcircle.
.smallcircle.
.smallcircle.
4 B 5.7 5.0 5.4 .smallcircle.
.smallcircle.
.smallcircle.
5 C 7.7 7.5 7.6 .smallcircle.
.smallcircle.
.smallcircle.
6 D 8.1 7.0 7.6 .smallcircle.
.smallcircle.
.smallcircle.
7 E 8.7 7.5 8.1 .smallcircle.
.smallcircle.
x
8 E 7.2 6.2 6.7 .smallcircle.
.smallcircle.
.smallcircle.
9 F 7.8 7.2 7.5 .smallcircle.
.smallcircle.
.smallcircle.
10 B 9.0 7.9 8.5 x x x
11 E 9.3 8.5 8.9 x x x
12 G 7.7 6.9 7.3 x x x
13 H 8.2 7.7 8.0 x x x
14 I 7.7 6.7 7.2 x x x
______________________________________
*raw material: thickness 0.7 mm, outer diameter of pipe 55 mm.phi.-
*target machining shape: bellows having peak pitch of 15 mm, 18 peaks and
full length of about 250 mm
*peak height: 10, 11 and 12 mm (corresponding to figures in "machining
cracks")
Cracks did not at all occur in the steels Nos. 1 to 9 of the steels of the
present invention when they were formed into bellows having a peak height
of 10 mm. When the peak height was 11 mm, however, cracks occurred in the
steel No. 2 where the grain size measured in a section orthogonal to the
rolling direction and in the steel No. 7 where the mountain height was 12
mm. On the other hand, cracks occurred in the steels Nos. 10 and 11 where
the crystal grain size measured in a section (C section) in a direction
orthogonal to the rolling direction was great (the crystal size was
small). Oxides which were assumed to be Al oxides were detected in the
vicinity of the portions assumed to be the starting point, from the
observation of the fracture.
Cracks also occurred, during forming into bellows, in the steel No. 12
(steel No. G) having a lager amount of addition of Al. Considerably coarse
Al type inclusions were also detected in the portions which were assumed
to be the starting point of the fracture. The steel No. 13 (steel No. H),
having a high N content, had a high strength but low ductility, so that
cracks occurred. Cracks occurred during forming into the bellows in the
steel No. 14 (steel No. I) having a small amount of addition of Al.
Observation of the fracture revealed large quantities of Si oxide type
inclusions, and breakage was assumed to occur with these inclusions as the
starting points.
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